Proceedings of The Physiological Society

King's College London (2011) Proc Physiol Soc 22, C08

Oral Communications

A model for the electrical responses of human rod photoreceptors to the onset of steady backgrounds

O. A. Mahroo1, V. S. Ban1, B. M. Bussman1, H. C. Copley1, T. D. Lamb2,3

1. Department of Physiology, University of Cambridge, Cambridge, United Kingdom. 2. John Curtin School of Medical Research, Australian National University, Canberra, Australian Capital Territory, Australia. 3. ARC Centre of Excellence in Visual Science, Australian National University, Canberra, Australian Capital Territory, Australia.

The activation phase of a photoreceptor’s electrical response to light flashes has been modelled using biochemical analysis of phototransduction (Lamb & Pugh, 1992). This “LP model” has been successfully applied to a range of recordings from single cells (Lamb & Pugh, 1992) and living subjects (Cideciyan & Jacobson 1996). Here we investigated whether a simple modification of the equation would predict responses to the onset of steady backgrounds and tested this against electroretinogram recordings. Scotopic electroretinograms were recorded from three normal human subjects, using conductive fibre electrodes, in response to both light flashes (white or blue, ranging in intensity from 0.13 to 740 scotopic cd m-2 s) and steady backgrounds (white, ranging in intensity from 33 to 1900 scotopic cd m-2). The rod-system response was isolated by subtracting responses recorded in the presence of a blue rod-saturating background (38 scotopic cd m-2) from responses to identical stimuli recorded in the dark. Subjects’ pupils were dilated pharmacologically using 1% tropicamide. The standard “flash response” form of the LP model was applied to flash responses, as in previous studies, to obtain values for the three parameters: maximal response amplitude amax, amplification constant A, and delay time td. A new “step response” equation was derived using Eqn 6.6 of Lamb & Pugh (1992), as 1 - exp{ -I A (t - td)3 / 6 } where I is the step intensity. This equation provided a good fit to rod responses to steps of light using the same parameter-values as for the flash responses. The results support the applicability of the LP model to ERG responses, and we have derived a new expression to fit responses to onset of backgrounds. Diseases affecting photoreceptors have been shown to alter the parameters obtainable using the LP model, and this work will support future attempts to analyse the effects of such abnormalities more quantitatively in living subjects.

Where applicable, experiments conform with Society ethical requirements